It has already been proposed to install on the target a sensor of the mechanical vibrations to which the target is subjected when it is struck by a projectile, so as to obtain an electric signal which, after passing through an amplifier and a threshold device, operates a flip-flop connected to a counter. Hits on the target are distinguished from projectiles passing near the target, by means of a threshold device which only allows vibrations which have an amplitude greater than a given limit to affect the counter. But practice shows that this distinction is illusory since a projectile which glances on the target sometimes produces a vibration whose amplitude is greater than that of a vibration due to a direct hit on the target.
It has been observed, other things being equal, that impacts on a target produce vibrations with higher frequencies than the simple passage of projectiles close by. Some known devices have therefore been improved by inserting a high-pass filter in the sensor circuit.
However, the applicant has observed that it is not possible to discriminate satisfactorily between direct hits on the target and near misses of the target using only the amplitude characteristic in a frequency band of the signal coming from a vibration sensor. In order to detect hits with a very small proportion of errors, it has been found that the energy in a high frequency band of the signal given by the vibration sensor must be calculated.